# Creating Latitude Grid from DEM

I am trying to evaluate an equation in the raster calculator and I need latitude values. I have the DEM for the region and am wondering if it is possible to create a latitude raster based on the DEM?

Thanks a lot!

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What software... ArcGIS? Is your DEM projected? – Luke Aug 11 '11 at 12:14

Unproject the DEM to determine the geographic region it covers.

Create the latitude coordinate grid for this region. (In Arc* products prior to version 10, this is done with "\$\$YMap": http://forums.esri.com/Thread.asp?c=93&f=1740&t=182407.)

Projecting this grid produces the latitude raster. For the projection, use some form of interpolation for the resampling (bilinear should be good enough in most cases), not nearest-neighbor.

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You can still use the built in GRID variables/scalars (e.g. \$\$YMAP) in ArcGIS 10 - forums.arcgis.com/threads/18608#post106446 – Luke Aug 12 '11 at 7:34

whuber's method is simpler, but this should also work if you have Spatial Analyst and ArcGIS:

1. Spatial Analyst Tools > Extractions > Sample, to create a points layer for each cell
2. Add X and Y fields to the points layers and calculate the X and Y values using "calculate geometry"
3. Use Conversion Tools > To Raster > Points to Raster to create a new grid for each value.
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I realise this is an old thread but thought I'd contribute to help anyone searching in the future. My DEM was too large to use Patrick's method, so I used a points file that contained a reasonable number of points (approx. 70, spread across the extent of the United Kingdom), then interpolated between these (using latitude as the z value). Hope this is helpful to someone, sometime!

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I used Kate's approach in ArcGIS (very much appreciated, even after all this time). I used a basic IDW interpolation with a variable search window on ~300 points. The rabbit hole that led me this far has been trying to ADD a directional elevation trend to to an existing DEM dataset to impose a stronger flow accumulation trend in a flat landscape. As the \$\$YMAP/\$\$XMAP GRID variables are no longer supported in Arc 10+, this was the easiest workaround for me. Doing this is was able to create two rasters for X and Y coordinate values to then work into map algebra.

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This is now trivial to do with GRASS tools available within QGIS. Open your raster data in QGIS (e.g., 'raster_dem.tif') and open the GRASS tool `r.rastercalculator` in the Processing Toolbox. For the `formula` variable, enter this:

``````raster_dem = y()
``````

This will produce a raster of latitude values. If you also need a raster of longitude values, simply use `x()` instead of `y()`. It's much faster than previous methods too.

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